docs: def _spectrum(self, photon_energy):
          """Compute differential IC spectrum for energies in ``photon_energy``.
  
          Compute IC spectrum using IC cross-section for isotropic interaction
          with a blackbody photon spectrum following Khangulyan, Aharonian, and
          Kelner 2014, ApJ 783, 100 (`arXiv:1310.7971
          <http://www.arxiv.org/abs/1310.7971>`_).
  
          Parameters
          ----------
          photon_energy : :class:`~astropy.units.Quantity` instance
              Photon energy array.
          """
          outspecene = _validate_ene(photon_energy)
  
          self.specic = []
  
          for seed in self.seed_photon_fields:
              # Call actual computation, detached to allow changes in subclasses
              self.specic.append(
                  self._calc_specic(seed, outspecene).to("1/(s eV)")
              )
  
          return np.sum(u.Quantity(self.specic), axis=0)